1. Field of the Invention
This invention relates to a fluorescent substance to be employed in a light-emitting device and, in particular, to a fluorescent substance to be employed in a display device such as a fluorescent display tube (VFD), a PDP, a CRT, an FED, an SED and a projector, and in a light-emitting device where a blue-emitting diode or an ultraviolet-emitting diode is employed as a light source.
2. Description of the Related Art
An LED lamp utilizing a light-emitting diode is now employed in various kinds of display devices such as a portable instrument, PC peripheral equipment, OA equipment, various kinds of switches, a light source for a backlight and a display board. A high load LED is known to become hot as it is actuated, thus increasing the temperature of fluorescent substance up to about 100° C. and, due to this rise in temperature, the emission intensity of the fluorescent substance degrades. It is desirable that the fluorescent substance hardly degrade even if the fluorescent substance is caused to rise in temperature.
Incidentally, with regard to the development of a flat panel display device, although many efforts are now vigorously made on the development of a plasma display panel (PDP) and liquid crystal display (LCD), a field emission type display is expected to be more advantageous than the PDP or the LCD in the respect that it is capable of providing a clearer image.
The field emission type display is equipped with a screen where red, green and blue fluorescent substances are arrayed, and with a cathode which is disposed to face the screen at smaller intervals than those of the CRT. The cathode is provided with a plurality of electron sources as an emitter element, emitting electrons depending on a potential difference between the electron sources and the gate electrodes disposed near the electron sources. The electrons thus emitted are accelerated by an anode voltage (accelerating voltage) of the fluorescent substances side and to impinge against the fluorescent substances, thereby fluorescent substances emit light.
As for the fluorescent substances to be employed in the field emission type display constructed as described above, it is required that the fluorescent substances exhibits sufficiently high luminous efficiency, so that when the fluorescent substances are saturated with electric charge as excited by a high current density, they exhibit a sufficiently high luminous efficiency. The conventional sulfide fluorescent substances (ZnS:Cu, ZnS:Ag) which have been employed as fluorescent substances for a CRT may be a prospective candidates as meeting the aforementioned requirements. It is however reported that the sulfide fluorescent substances such as ZnS decompose under the conditions where a low energy cathode-ray display screen is excited. The decomposed matters thus generated would badly degrade a heat filament which emits an electron beam. In particular, ZnS-based blue fluorescent substances that have been conventionally employed are more vulnerable to luminance deterioration as compared with red fluorescent substances and green fluorescent substances, thereby raising the problem that the display color of a color image plane changes with time.